While ideally this should be implemented by the OS and tablet
driver, some applications implement their own tablet pressure
curve support. This allows tweaking based on task, and work-
arounds for missing OS-level options.

A full custom curve like in Krita is probably overkill for
Blender, so this adds two simple options:

Pressure required to achieve full brush intensity.

Softness control, using a gamma curve internally.

The important one is the max pressure setting, because it is
critical for ergonomics, but the Linux Wacom driver lacks it.

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It seems Grease Pencil brushes support per-brush full pressure curves, so this is one more argument to keep the global options simple. Perhaps detailed curves should also be implemented for regular painting and sculpting brushes.

For grease pencil I guess those are more artistic choices about how the brush should behave, or at least they should be if they are not in the user preferences. Having some simple options like this in the user preferences seems fine.